The present invention relates to a burner for use in a partial combustion process for producing synthesis gas from a finely divided solid fuel, such as pulverized coal. The invention further relates to a process for the partial combustion of a finely divided solid fuel in which process such a burner is used.
The generation of synthesis gas is achieved by the partial combustion, also called gasification, of a hydrocarbonaceous fuel with free-oxygen at relatively high temperatures. It is well known to carry out the gasification in a reactor into which solid pulverized fuel and free-oxygen containing gas are introduced either separately, or premixed at relatively high velocities. In the reactor a combustion process is maintained in which the fuel reacts with the free-oxygen at temperatures above 1000.degree. C. The solid fuel is normally passed together with a carrier gas to the reactor via a burner, while free-oxygen containing gas, such as pure oxygen or oxygen-rich air, is introduced into the reactor via the same burner either separately or premixed with the solid fuel. Since solid fuel, even when it is finely divided, is normally poorly reactive, great care must be taken that the reactants, the fuel and the free-oxygen, are effectively mixed with one another prior to or during the combustion process. Inadequate mixing of the reactants will result in the generation of a product gas with a varying constituency, which is caused by the fact that parts of the fuel receive insufficient oxygen for a proper gasification in the time available, while other parts of the fuel receive too much oxygen, so that in the latter case the fuel is completely converted into less valuable end products, viz. carbon dioxide and water vapor. Inadequate mixing of the reactants has another important disadvantage in that zones of overheating are generated in the reactor which zones might cause damage to the internal refractory lining of the reactor and/or the applied burner(s).
In order to attain a sufficient mixing of solid fuel with oxygen it has already been proposed to mix the fuel and oxygen in or upstream of the burner prior to introducing the fuel into the reactor. This implies, however, a disadvantage in that--especially at high pressure gasification--the design and operation of the burner are highly critical. The reason for this is that the time elapsing between the moment of mixing the fuel with oxygen and the moment the fuel/oxygen mixture enters into the reactor zone must be invariably shorter than the combustion induction time of the mixture. The combustion induction time shortens, however, at a rise in gasification pressure and as burner size increases. If the burner is operated at a low fuel load or, in other words, if the velocity of the fuel/oxygen mixture in the burner is low, combustion of the fuel/oxygen mixture may easily take place in the burner itself, which would result in overheating and the risk of severe damage to the burner.
The above problem of premature combustion of the fuel in the burner itself can be overcome by mixing the fuel and oxygen outside the burner in the reactor zone itself. In the latter case, special steps should, however, be taken to obtain a good mixing of fuel with oxygen, necessary for a proper gasification of the fuel.
Various designs have been made in the past in an attempt to provide a burner which produces during operation a substantially uniform mixture of solid fuel with oxygen in the reactor space. These burners are normally of the so-called axisymmetric type, i.e., which produce essentially axisymmetric flows of fuel and oxygen during operation, and which employ mainly the momentum of the oxygen flow to break up the flow of solid fuel. In these burners the solid fuel is normally transported through a centrally arranged channel while the oxygen is supplied at an angle to the issuing coal flow. Use of the momentum of the oxygen flow for breaking up the core of solid fuel is, however, limited by the maximum allowable oxygen velocity in the burner above which friction-induced ignition of the burner material might occur. A further limitation of the momentum of the oxygen flow is set by the maximum throughput of oxygen which is constrained by requirement of efficient gasification of a particular type of solid fuel at a given load factor. Axisymmetric injection of solid fuel and oxygen into a reactor can therefore lead to unburned solids resulting in a conversion loss and thus a reduction of the efficiency of partial combustion.
Apart from a loss in the rate of conversion insufficient break-up of the coal flow can further lead to blockage of the reactor-slagtap due to unburned or insufficiently burned solids and/or contamination of the product gas with fine particles of unconverted fuel. This particularly applies to reactor geometries where the slagtap and/or the product gas outlet are placed symmetrically with respect to the main flow axis or with respect to the burner axis.
An object of the present invention is to overcome the above problem of insufficient breaking up of the solid fuel flow resulting in conversion losses, blockage of the reactor outlet and/or contamination of the product gas.